[9] In March 2019, McMahon won preselection to replace Nigel Scullion as the Country Liberal Party's lead Senate candidate at the 2019 federal election.
[17] On 8 April 2022, McMahon announced she had joined the Liberal Democrats, and would run on the party's Northern Territory Senate ticket at the May federal election.
[18][19] The Liberal Democrats won approximately 9% of the vote in the Northern Territory Senate race, not enough for McMahon to retain her seat in the chamber.
[20] In March 2020, McMahon argued that a nuclear power plant should be built in northern Australia to lower emissions and improve the country's economy.
This would include a partial repeal of the Euthanasia Laws Act 1997, allowing the Legislative Assembly to legalise assisted suicide.
[22] In November 2021, McMahon was one of five Coalition senators who voted against the government in support of One Nation’s COVID-19 Vaccination Status (Prevention of Discrimination) Bill 2021.